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Thread: MK1 vibration from front and bump steer..?

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    MK1 vibration from front and bump steer..?

    I have a MK1 and I'm getting a vibration from the front end at highway speeds. I jacked the front up and checked for loose/ worn components but everything seemed tight. I think it might be my tires which are shockingly 17 years old! The car has always been garaged so no dry rot and they only have 17000 miles on them still I think I need new tires. They are BFG's. I'm thinking I will pull them and take them to be speed balanced which should show me if they are the source of the vibration right?
    The car is set to go away for the Winter so it doesn't make sense to buy new tires now just to have them sit until the Spring.

    Another issue that has bothered me for many years is the bump steer. I spoke with a guy that knows the MK1 in and out and he says that this was a known problem with them. The question is will a bump steer kit help the wandering? I see that FFR is having a 25% off sale and they have the kit.
    Will installing the bump steer kit provide a noticeable improvement?

  2. #2

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    Tires get hard and out of round over time. And BFG's are not that great of a tire to begin with. If yours are 17 years old, they are waaaaaaay out of date. They need to go. Your next trip in that car needs to be to the tire shop.

    As for bump steer, this comes up a lot. Do you need a bump steer kit? I don't know. Do you have bump steer? Have you ever measured it? If you've never measured it, you'll never know. If you install the kit without measuring anything, you have a great chance of making things a lot worse. This set up isn't fancy. But it's inexpensive, accurate, and repeatable.

    .boB "Iron Man"
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    BDR 1642: Coyote, 6 Speed Auto, Edelbrock Supercharger
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    Seasoned Citizen NAZ's Avatar
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    I can't stress enough that driving on 17-year old tires is a hazard to you, your passengers, and everyone around you. Tires have a shelf life and your tires are at least a decade past that usable life. First thing to do is get new tires and I suggest you do not drive the car to the tire shop. While you're at it, learn to read the date codes on the tires. Since I never deal with whitewalls I always have the tire mounted with the date codes to the outside (inside on inner duals) so they're easy to see. Use the 6-year rule on tires and you will mitigate the risk of old tires failing. When tires fail at speed it can cause damage, injury, and death. If you live in one of the litigation crazy states you might also think about how your culpability will be judged if your old tires are deemed a contributing factor to an injury traffic collision. Old tires are a ticking time bomb.

    As for bump steer -- see my suggestion on your other post. Remember, you can't control what you don't measure. You need a bump steer gauge to properly set-up a bump steer kit.

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    It is crazy how the years have ticked by and the age of the tires have been on my mind the last few years but it is hard to justify buying new ones when they still look good. I just put new tires on our Wrangler because the old ones were dry rotted. You guys are right that these old tires on my car are a hazard and it is only a matter of time before I have a problem.
    Back in the day FFR recommended BFG's. What would you recommend for a 15 inch wheel today?

    It sounds like the bump steer needs to be investigated more but the first thing is new tires.

  5. #5

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    Quote Originally Posted by Todd Haskins View Post
    What would you recommend for a 15 inch wheel today?
    Do a search here and on the other board. You'll find a couple of recommendations, but not many. Choices in 15" tires are extremely limited. It might be time to upgrade to 17" or 18" wheels.
    .boB "Iron Man"
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    BDR 1642: Coyote, 6 Speed Auto, Edelbrock Supercharger
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    Bumpsteer results in a toe change. You need to measure the toe of the front wheels at different suspension levels, a gauge at the front and rear of the spindle. The wheel moves in an arc, in and out, track width changes in that arc therefore measuring at just the front will give a false reading. Cant correct it without know what it is before. There are rack extenders, lowering bushings, and tierod ends that allow placement to be set at the spindle. May take a combination of these to correct it.

  7. #7
    Not a waxer Jeff Kleiner's Avatar
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    Quote Originally Posted by Todd Haskins View Post
    ...What would you recommend for a 15 inch wheel today?
    I built a Mk4 with 15s for a customer and he chose Cooper Cobra GT:

    http://us.coopertire.com/tires/cobra-radial-g-t.aspx

    Not a high performance tire (no 15s short of a pure track tire are) but they have suited him well for street cruising.

    Cheers,
    Jeff

  8. #8
    Senior Member CraigS's Avatar
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    Here is my technique for checking bumpsteer. A dot laser would be easier but I happened to buy this and just have to assure the beam is vertical.
    IMG_20130428_162158_880 by craig stuard, on Flickr
    My first check had the laser move maybe 3 inches along the side of the box. Usually the first change is to lower the rack by using offset aluminum rack bushings. But I have also had luck w/ using the bump steer kit to move the tierod to the top of the steering arm instead of moving the rack. You can also get some bump steer change by changing caster since this will move the end of the steering arm up or down. BTW I happened to have the coilover removed when I took that pic but you only need to undo the top mount bolt. Than guide the top of the coilover so it doesn't bind on anything as you run the spindle up and down.
    Last edited by CraigS; 11-24-2017 at 03:39 PM.
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

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    Senior Member Big Blocker's Avatar
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    I have to agree with the posters that say; how do you know you need it if you haven't measured it??
    Unfortunately, so many builders just throw money at the car because someone said "all the early cars suffered from bump steer" . . . NOT TRUE at all.

    Get it measured and then see if a "bump steer kit" will correct your issue. You may find that rack bushings or rack tie-rod extenders are the answer. Bump steer is a floating issue that changes with ride height, rack position, pivot points of the tie-rods and a few other things thrown in for good measure. Get it measured and verified and then read all you can about how to correct it.

    FWIW, BFG's get rock hard with age and are dangerous. What you may not be able to see is what's going on inside the tire at that age, even if the outside "looks great". Get them replaced ASAP.

    Doc
    FFR3712K (MKII) in Lost Wages Nevada.
    5.0 w/tubular GT-40 EFI, E303 cam, Custom 4 into 4 headers, T5, 3-Link 3.73 rear. Full F5 tubular suspension. Drop Butt mod, Dash forward mod, custom foot box air vents, custom turn signal system. 13" PBR brakes, Fiero E-Brake mod, Flaming River 18:1 rack w/ F5 bump steer kit on Breeze bushings. 17" Chrome Cobra "R's" w/ 275 fronts and 315 rears. MKIV seats. FORD Royal Blue w/ Arctic White stripes.

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    Senior Member rich grsc's Avatar
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    Quote Originally Posted by Big Blocker View Post
    I have to agree with the posters that say; how do you know you need it if you haven't measured it??
    Unfortunately, so many builders just throw money at the car because someone said "all the early cars suffered from bump steer" . . . NOT TRUE at all.

    Get it measured and then see if a "bump steer kit" will correct your issue. You may find that rack bushings or rack tie-rod extenders are the answer. Bump steer is a floating issue that changes with ride height, rack position, pivot points of the tie-rods and a few other things thrown in for good measure. Get it measured and verified and then read all you can about how to correct it.

    FWIW, BFG's get rock hard with age and are dangerous. What you may not be able to see is what's going on inside the tire at that age, even if the outside "looks great". Get them replaced ASAP.

    Doc
    So true!!!

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    Seasoned Citizen NAZ's Avatar
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    Bump steer and roll steer are two different things that can cause un-commanded steering inputs. Unless you have a sensitive butt you could very well be experiencing roll steer and believe it to be bump steer. With the many posts I read about setting ride height it's clear to me that many folks out there have no clue that arbitrarily changing ride height without regard to the lower control arm (LCA) angle can result in some nasty handling issues, especially on a very short wheelbase car with relatively short LCA.

    The front and rear suspensions act as a system. They must work in concert to result in good handling, sufficient traction, and good ride qualities. To properly set-up a suspension system you have to understand how each part works and how it affects other parts of the system. And as I stated above, you can't control what you don't measure. And sometimes you may not even be attacking the root cause of a handling issue if you are not measuring the correct end of the car.

  12. #12
    Senior Member Big Blocker's Avatar
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    Just some thoughts to consider . . .

    Quote: "Bump steer or roll steer is the term for the tendency of the wheel of a car to steer itself as it moves through the suspension stroke. It is typically measured in degrees per foot."

    "Bump steer causes a vehicle to turn itself when one wheel hits a bump or falls down into a hole or rut. Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads. For example, if the front left wheel rolls over a bump it will compress the suspension on that corner and automatically rotate to the left (toe out), causing the car to turn itself left momentarily without any input from the steering wheel."

    "The linearity of the bump steer curve is important and relies on the relationship of the control arms and tie rod pickup points, and the length of each part. As the suspension goes through bump and droop, each part follows an arc resulting in a change of effective length."

    "During a bump, both wheels rise together. When rolling as the car leans during a curve, the inside suspension extends and the outside suspension compresses. Typically this produces "toe in" on one wheel, and "toe out" on the other, thus producing a steering effect.

    Cars with rear live axles, also known as solid axles, tend to not have true bump steer. Since both wheels are connected to a single, rigid member they are incapable of having any toe angles under normal conditions."
    FFR3712K (MKII) in Lost Wages Nevada.
    5.0 w/tubular GT-40 EFI, E303 cam, Custom 4 into 4 headers, T5, 3-Link 3.73 rear. Full F5 tubular suspension. Drop Butt mod, Dash forward mod, custom foot box air vents, custom turn signal system. 13" PBR brakes, Fiero E-Brake mod, Flaming River 18:1 rack w/ F5 bump steer kit on Breeze bushings. 17" Chrome Cobra "R's" w/ 275 fronts and 315 rears. MKIV seats. FORD Royal Blue w/ Arctic White stripes.

  13. #13
    Senior Member Packer fan's Avatar
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    The vibration could also be a broken belt on one of the tires.
    And that will not be corrected with a balance.

  14. #14
    Seasoned Citizen NAZ's Avatar
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    Big Blocker has provided us with a fair explanation of bump steer but nothing about roll steer.

    So let me try to explain how a solid rear axle can cause roll steer for folks new to this concept. The lower control arms (LCA) control this on three and four link suspension systems. The LCAs travel in an arc with the fixed end mounted to the chassis (at least we'll use that concept for this explanation). Traveling in an arc means that the wheelbase actually changes as the suspension goes through the extreme ends of bump and rebound. See the attached cartoon for a graphic example.

    When a car takes a corner hard the rear rotates around the roll center. We won’t get into roll center or roll axis – folks interested can Google it. For a typical FFR 3-link rear that roll center is established by the panhard bar and causes the LCA on the inside of the corner to angle up at the front and the outside LCA to angle down from the angle at static ride height. This changes the wheelbase on each side independent of the other side as each LCA travels through its arc. If both LCAs are set parallel to the ground at static ride height (which is also the longest wheelbase position) then each travel in virtually the same but opposite arcs and the wheelbase on each side changes (is shortened) by the same amount. However, if the LCAs are angled either up (at the front remember) or down then each side will travel through a different arc and the wheelbase change will be different from side to side. One side will be longer than the other. This causes the rear axle to skew in the chassis along with an un-commanded steering input. Much like how a forklift steers.

    So the moral of this story is if you change rear ride height with no regard to how that affects the LCA angle you could be setting up your car for “roll steer”. If you have a sensitive butt you will not only feel (and see) the car turn but will feel it yaw as well. Roll steer can be misidentified as bump steer coming from the front suspension and with all the different tire sizes out there all set to the same ride height I suspect there are a lot of cars out there with some degree of roll steer.
    IMG_0752.JPG
    Last edited by NAZ; 11-29-2017 at 02:24 PM.

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    Update. I bought tires and just took them for a test drive and the vibration seems to be gone at least up to 50mph. I didn't test it on the highway where the vibration was bad between 65-70.
    I think the car handles better now with the new tires. Looking at the BFG's data they were 18 years old and they still looked good but I think they were getting hard as another poster said BFG's do.
    Still 17 years on the car. I don't know what I was thinking driving around on such old tires. I guess I thought that with only 17,000 miles and always garaged that they were still good.

    Another thing that has been greatly reduced is hitting a bump and having the car lurch but I'm sure there are still fine tunings that can be done to make it even better.


    I bought the Cooper Radial GT's that Jeff K recommended and I got a good deal. I called up my local Town Faire Tire and they gave me a price. I then told them that I found them cheaper at Tire Rack so they matched that price plus some then when I brought the old tires/ wheels in to swap, I showed them how I found the Coopers even cheaper at another online tire store and they knocked another $80 off.

    I'm so glad I got these tires and I'm thankful that I didn't have a tire failure. The only problem is now the car will be going away to storage for the Winter.

    Thanks guys.

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    Senior Member phileas_fogg's Avatar
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    That's cool that your local store did you right. I like buying local; I figure that instead of paying for some CEO's third house I'm paying for a local kid's dance lessons, summer camp... You get the idea.


    John
    MK IV Roadster #8631
    Ford 302, Holley Terminator EFI, T5z, 3.55 Rear End, IRS, 17” Halibrand Replicas (9” front, 10.5” rear), Nitto 555 G2’s (275/40ZR17 front, 315/35ZR17 rear), Fast Freddie’s Power Steering, F5 Wilwood Brakes, FFMetal’s Firewall Forward, Forte’s Hydraulic Clutch & Throttle Linkage
    https://www.ffcars.com/threads/phile.../#post-4776313

  17. #17
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    Quote Originally Posted by phileas_fogg View Post
    That's cool that your local store did you right. I like buying local; I figure that instead of paying for some CEO's third house I'm paying for a local kid's dance lessons, summer camp... You get the idea.


    John

    Actually Town Fair Tire is a huge chain so I guess I did help that CEO buy another house.
    Last month I had to get new tires for my wifes Wrangler and I tried to go local and they were pricey. They tried to sell me no name tires that I later found out had terrible reviews for more money then Town Fair eventually sold me Good Year Wranglers for.

    A few years back I had to buy tires for my truck and I did a huge search for the best price for the BFG KO2's. The local shops hovered around $1000, Sears was $1300 and wouldn't price match. TF Tire price matched the best deal I could find plus a rebate and I was driving out the door for $850.
    Buying local is great and I don't mind paying a bit more but not so when they are going to gouge.

    It is a great comfort to have these new tires and now I want to drive my Cobra ! but it is a bit chilly with temps in the mid 40's today.

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